Ford’s decision to bring development of a new continuously variable transmission for its line of plug-in hybrid and hybrid vehicles in house wasn’t due to dissatisfaction with Aisin, the Japanese supplier that provided previous-generation hybrid CVTs.
Rather, Ford engineers thought they could make an even better transmission while cutting costs.
The new Ford-developed HF35 transmission, like the Aisin-sourced unit, is an electrified CVT. But it outperforms its predecessor in several key areas, says Shailesh Kozarekar, manager-transmission electronics.
“We made improvements to the gear-ratio operation and the match between the engine, and that gave us a lot more flexibility to optimize the (transmission) to get the best fuel economy and other big improvements over the outgoing product,” he tells WardsAuto.
The new HF35 hybrid transmission will be used in the upcoming C-Max hybrid and plug-in hybrid cross/utility vehicles, as well as the Fusion PHEV and hybrid and the Lincoln MKZ hybrid.
One of the benefits of the new CVT is an electric-only top speed of 85 mph (137 km/h) in the upcoming Ford C-Max Energi PHEV, besting the outgoing Escape Hybrid’s 44 mph (71 km/h).
Fuel-economy benefits are hard to quantify, as Ford hasn’t offered a PHEV before. But the C-Max hybrid, which also uses the HF35, achieves 47 mpg (5.0 L/100 km) city/highway, besting the Escape Hybrid’s 34 mpg (6.9 L/100 km).
It’s the unique design of the HF35 that allowed for such improvement, Kozarekar says, noting the electrified CVT differs from traditional CVTs in many regards.
A traditional CVT has two pulleys, one attached to the engine and the other to the wheels. A belt running between the pulleys is made shorter or longer by compressing or relieving the pulleys, which creates continuously variable gear ratios.
Many hybrids utilize CVTs rather than a traditional step transmission because they offer an infinite number of gear ratios, therefore offering optimized fuel economy in most situations.
The electrified CVT replaces the pulleys with motors, one of which is connected to the wheels and the other to a planetary gear set. The gear set has three connections, one to the engine, one to the wheels and one to a second electric motor, called a generator motor.
“Because of this unique (setup), we can form a unique planetary arrangement,” Kozarekar says. “We can change the (electric) motor speed and choose any speed we want to operate the engine.”
The great degree of flexibility enables the engine and transmission to run at the most efficient points because the control software can choose the optimal electric motor and gear ratio combinations.
Kozarekar says Ford considered using a traditional CVT in the new hybrid vehicles, but determined an electrified version was the way to go despite its greater complexity.
“In our estimate, the electrical CVT, with the right motor, engine and mechanical operations, is more efficient than a CVT,” he says, noting it took about three years to develop the HF35.
Building the transmission in-house, rather than sourcing it from Aisin, shaved 20% of the development costs, Ford says.
It also allowed engineers to have greater control over the development of other electrified-vehicle systems.
“A hybrid system is quite complex, so having the control we have gives us a better chance of optimizing the entire system,” Kozarekar says.
